Most aluminum is produced by the Hall-Heroult process which involves the electrolysis of alumina dissolved in molten cryolite (Na.sub.3 AlF.sub.6) at about 950.degree.-980.degree. C. using carbon anodes which are consumed with the evolution of CO/CO.sub.2. However, the process does suffer from major disadvantages. The high cell temperature is necessary to maintain alumina in solution, but requires heavy expenditure of energy. At the high cell temperature, the electrolyte and the molten aluminum aggressively react with most ceramic materials, and this creates problems of containment and cell design. The anode-cathode distance is critical; and since the anodes are continually being consumed, this creates problems of process control. Further, the back oxidation of Al to Al.sup.3+ decreases the current efficiency.
Potentially the electrolysis of alumina in NaF-AlF.sub.3 melts at "low" temperatures has several distinct advantages over the conventional Hall-Heroult process operating at about 960.degree. C. Most important are higher current and energy efficiencies and the possibility of designing a completely enclosed electrolytic cell.
Problems which hindered the practicability of low temperature electrolysis so far are the low alumina solubilities in low bath ratio electrolytes, as well as low alumina solution rates. Under these conditions, the transport of oxide ion species in the electrolyte to the anode surface can not be maintained at the anode current densities normally used in conventional Hall-Heroult cells. The configuration of such cells and the utilization of consumable carbon anodes do not permit a substantial variation of the relative surface area of anode and cathode.
Low temperature alumina electrolysis has been described in U.S. Pat. No. 3,951,763 and requires numerous expedients such as the use of a special grade of water-containing alumina to protect the carbon anodes, and the bath temperature had to be 40.degree. C. or more above the liquidus temperature of the Na.sub.3 AlF.sub.6 /AlF.sub.3 system in an attempt to avoid crust formation on the cathode. The practical realization of this process, as described in an article "Bench Scale Electrolysis of Alumina in Sodium Fluoride-Aluminum Fluoride Melts Below 900.degree. C." by Sleppy and Cochran (inventors of U.S. Pat. No. 3,951,763) and published in "ALUMINUM" 1979.9 p. 604-606 reveals, however, that the carbon anodes were severely attacked during anode effects accompanied by excessive CF.sub.4 emissions. Crusts also formed on the cathode up to electrolyte temperatures of 930.degree. C.
The formation of cryolite crusts on the cathode was caused by depletion of aluminum containing ions at the cathode and a consequent shift in the bath composition at the cathode interface to high NaF content. According to the phase diagram of the NaF-AlF.sub.3 system, the decrease in AlF.sub.3 content need be only 2% at 860.degree. C. with a bath weight ratio of 0.8 before cryolite will precipitate at the cathode. However, if the same bath is employed at 930.degree. C., 100.degree. C. above the liquidus temperature, the local decrease in AlF.sub.3 has to be greater than 7% before cryolite precipitates.
Attempts to reduce the temperature of alumina electrolysis in fluoride baths have thus been unsuccessful. Because of the difficulties encountered with low temperature alumina-containing melts, major efforts to secure the advantages of "low" temperature electrolysis were devoted to using different electrolytes, notably chloride based electrolytes where the anodic reaction is chlorine evolution. See e.g. K. Grjotheim, C. Krohn and H. Oye, Aluminium 51, No. 11, 1975, pages 697-699. However, problems related to the production of pure AlCl.sub.3 have hitherto eliminated this process from commercial application.
Another route of producing aluminum in a "low temperature" process was considered by W. E. Haupin in an article published in "Light Metal" Vol 1979, p. 356-661. This method comprises dissolving Al.sub.2 O.sub.3 in an LiCl/AlCl.sub.3 electrolyte, whereby Al.sub.2 O.sub.3 and AlCl.sub.3 form AlOCl which is electrolyzed at approx. 700.degree. C. However, the author reports that the rate of aluminum production is too low for practical commercial application.